The present invention relates to switches for use in electronic circuits and, in particular, to switches in which the movement of a flexible membrane closes the switch contacts.
Membrane switches of the mechanical type, in which movement of a flexible membrane simultaneously causes the movement of a conductive member to bridge and close the normally open switch contacts, are well known in the art. Membrane movement is typically provided by light finger pressure which moves an internal conductive member through a small open gap to close the switch.
In the construction of one type of membrane switch, one or both of the switch contacts is incorporated into an insulative substrate which may comprise a film base or a circuit board. The underside of an upper flexible membrane, which overlies the substrate and is spaced from the contact(s) thereon, includes a conductive member which may be the other of the switch contacts or a conductive bridge, either of which is adapted to close the contacts upon depression of the flexible membrane. In either case, there is a small air gap or space between the contact area of the substrate and the conductive underside of the flexible membrane in the normally open position of the switch. Such constructions are shown, for example, in U.S. Pat. No. 3,898,421 and 3,988,551.
Depression of the flexible membrane to close the switch will compress the air or other gas occupying the space between the membrane and substrate. If the air or gas is not allowed to escape, the internal pressure build-up may adversely affect the ease of operation of the switch. Since membrane switches are often used in multiple array in a keyboard, one means of allowing the compressed air or gas to escape is to vent each space to the other switches in the array through a series of interconnected internal venting channels, as disclosed, for example, in U.S. Pat. No. 3,995,126. The switches in this type of keyboard array are sealed from outside air. However, a pressure differential may still develop between the outside air and the air or other gas sealed in the spaces within the switches, particularly through thermal cycling in operation or normal ambient temperature changes. This can result in a "dimpling" of the membrane and inadvertant closing of a switch.
One obvious and well known means of preventing the development of a potentially harmful pressure differential is to vent the internal air spaces within the switches to the atmosphere. However, certain desirable materials useful in the conductive contacts and bridging elements are reactive with ordinary contaminants in the outside air and susceptible to corrosion or tarnishing. The formation of a corrosive or tarnish layer on the switch contacts can cause unreliable operation, unacceptably high resistances and, in the worst situation, may insulate the contacts completely.
Attempts have been made to reduce the formation of tarnish and corrosion on the switch contacts by controlling the venting so that only enough outside air sufficient to equalize the pressure is allowed to enter. Such attempts have only been successful in reducing slightly the tarnish rate. Furthermore, as the vents to the outside are reduced in size to restrict the amount of air entering, they become more susceptible to plugging by dust and other airborne particles.